1. Field of the Invention
The present invention relates generally to asynchronous transfer mode (ATM) networks and more particularly to a cell exchanging system for relaying cells and exchanging cells at a high speed.
2. Description of the Prior Art
A number of ATM techniques and architectures have been proposed to switch voice data, video data, and other kinds of data. The ATM techniques are designed for use in a digital network such as an integrated services digital network (ISDN). ATM techniques improve the utilization efficiency of transmission in switching by statistical multiplexing of fixed length packets of the data, known as cells, on a broad band transmission line. The architectures for practicing ATM techniques include switching architectures for switching cells through the network.
FIG. 11 is a block diagram showing the change-over system of an ATM switch, which is described in Japanese Patent Lain-Open No. Hei 3-26038. The ATM switch is used to direct cells through the ATM network. As shown in FIG. 11, this ATM switch is provided with two ATM switch systems #1 and #2, which have the same structure. Redundant switch systems are provided to enhance the robustness of the network.
Identical ATM cells are input in parallel to the respective switch systems #1 and #2. These ATM cells are input in parallel to ATM switch buffers 30a and 30b of the ATM switch systems #1 and #2, and each cell is output to an external system after a delay. In order to monitor the number of cells which are stored in the ATM switch buffers 30a and 30b, ATM buffer cell counters 31a and 31b are provided. Each of the ATM buffer cell counters 31a and 32b increments its count value each time an ATM cell is input to the corresponding ATM switch buffer 30a and 30b, and decrements its count value each time an ATM cell is output from the corresponding buffer. In this way, the ATM buffer cell counters 31a and 31b always monitor the number of cells that are stored in the respective ATM switch buffers 30a and 30b.
Systems #1 and #2 are provided with respective difference detectors 33a and 33b, for comparing the count values of the ATM buffer cell counters 31a and 31b. Systems #1 and #2 are also provided with respective dummy cell markers 32a and 32b for writing dummy cells into the ATM switch buffers 30a and 30b, in accordance with the control signals sent from the difference detectors 33a and 33b.
A plurality of ATM switches are disposed at intersections (cross points) of input and output paths so as to form a crossbar type ATM exchanging apparatus.
The operation of the conventional ATM switch systems of FIG. 11 will be explained below with reference to FIGS. 12A-12C, 13A, and 13B. In FIGS. 12A-12C, 13A, and 13B, switch systems #1 (see FIG. 11) functions as the "currently used" system, whereas switch system #2 (see FIG. 11) functions as a "spare" system. In this illustrative case, each of the ATM switch buffers 30a and 30b stores the same cells "1"-"2", as shown in FIG. 12A. Accordingly, each of the ATM buffer cell counters 31a and 31b has a counter value of "4".
If ATM switch buffer 30b of switch system #2 ceases operating due to some problem, tile contents of the ATM switch buffer 30b are lost, and the count value of the ATM buffer cell counter 31b is reset to "0", as shown in FIG. 12B. Even if the ATM switch of system #1 becomes operational again, a certain amount of time must elapse before switch system #2 may assume the "currently-used" role previously served by switch system #2.
When switch system #2 assumes the "currently-used" role, difference detector 33b (FIG. 11) detects the difference between the count values of the ATM buffer cell counters 31a and 31b. Difference detector 33b causes the dummy cell marker 32b to generate dummy cells (indicated by "0" entries) which are stored in the ATM switch buffer 30b until the difference in count values is eliminated. Dummy cells are, thus, stored until there is no difference between the count values of the ATM buffer cell counters 31a an 31b, and hence the number of cells stored in the ATM switch buffer 30a equals the number of cells stored in the ATM switch buffer 30b.
For example, when cells "3"-"8" are stored in the ATM switch buffer 30a of switch system #1 and only the cell "8" is stored in the ATM switch buffer 30b of switch system #2, as shown in FIG. 12C, the difference between the numbers of cells stored in the respective switch buffers is "5". Hence, five dummy cells (indicated as "0") are generated and stored in the ATM switch buffer cell 30b. Both ATM buffer cell counters 31a and 31b then have a count value of "6".
In this state, switch system #2 begins to operate in the same way as switch system #1. FIG. 13A shows a subsequent state in which the leading four cells in buffers 30a and 30b have been output and in which four new cells have been supplied to the ATM switch buffers. FIG. 13B shows the state in which two additional cells (six cells in total) have been supplied to the ATM switch buffers 30a and 30b and the leading three cells have been removed from the buffers 30a and 30b. In the state shown in FIG. 13A, since the dummy cells remain, switch system #2 cannot function as a "spare" system. In other words, switch system #2 cannot be called upon to assume the role previously served by switch system #1. However, in the state shown in FIG. 13B, since no dummy cells remain, switch system #2 can be changed over to assume the role of system #1.
With the conventional cell exchanging system having the foregoing configuration, one control unit is shared by two switch systems so as to control which of a plurality of ATM systems for outputting cells to the same output path should output a cell. In such a case, the status of cells stored in the switch buffers are the same for the two switch systems. There is however a problem that the control units cannot be doubled.
On the other hand, when a separate control unit is provided for each of the switch systems 1 and 2, there might be a case in which the switch buffers for the ATM switch systems 1 and 2 might store different numbers of cells. In such a case, the systems 1 and 2 cannot be selected optionally.